Japanese non-examined laid-open publication JP2004-508213 discloses a bit holder in an impacting tool, wherein a steel ball is provided for holding the tool bit which is inserted in a bit inserting hole of the tool holder. As to the known bit holder, a long hole that goes through the radial direction in the tool holder is arranged for a prescribed length in the longitudinal direction and, in the aforementioned long hole, the steel ball is arranged so that it can move in the longitudinal direction and radial direction of the tool holder. As the tool bit is inserted into the bit inserting hole, the steel ball is pressed by the tip of the tool bit, so that it goes against the energizing force of a compressive coil spring to push a holding sleeve back to the depth side (the tool's main body side) in the longitudinal direction. It then moves outward in the radial direction so that it allows further insertion of the tool bit. As the tool bit is inserted to the prescribed position, the steel ball moves inward in the radial direction and is locked in a slot of the tool bit. In this locked state, the state is kept by a lock ring arranged appropriately to ensure that it covers the steel ball from the outer side in the radial direction. The lock ring can move freely in the longitudinal direction together with an operation sleeve that allows manual manipulation and, as the lock ring is moved from the position where it covers the steel ball to a recessed position, locking of the steel ball with respect to the slot on the tool bit is released, so that the tool bit can be pulled out from the bit inserting hole.
For an impacting tool that carries out a hammering operation or hammer drilling operation or other processing operation by the tool bit on concrete or other workpiece, dust is generated in the processing operation. Consequently, the bit holder arranged in the tip region of the impacting tool is always exposed to the dust. Consequently, it is necessary to adopt measures against dust to prevent dust from invading into the interior of the bit holder. However, for the conventional bit holder, there is still room for improvement with respect to the dust-proof measures.